Known in the art is a method for measuring mass flow of a substance based on the differential pressure across on orifice plate (diaphragm) installed in a pipeline in which the substance flow is measured, and on the density of the substance which to be measured (cf. V. G. Tseitlin. Techniques for Measuring Flow and Quantity of Liquids, Gases and Vapours (in Russian). 1968, pp. 27--40).
The need to measure the density of a substance complicates the method for determining the flow rate and lowers the accuracy, while the range of measured flow rates is restricted as there is a square relationship between the pressure difference and substance flow.
Known in the art is a method for measuring the flow rate of a substance by providing an auxiliary flow thereof in the form of a jet emerging in the direction at a right angle to the measured flow and intersecting a part of the cross-section of the measured flow. The differential pressure is measured at the points of the measured flow located opposite to the orifice from which the jet emerges, in the zone of influence of the jet (cf. P. P. Kremlevsky. Flow Meters and Quantity Counters (in Russian). 1975, pp. 645, 646).
This method also has a restricted range of measured flow rates of substance. In addition, either the velocity of measured flow or mass flow with known and constant density of substance in the flow can only be determined by this method. Measuring of mass flow of a substance with variable density, as in flow meters equipped with membranes, involves measuring the density too.
Known in the art is also a method for measuring mass flow of a substance based on the differential pressure between two points of the flow being measured, the pressure being created in a pipeline in the zone of a cylinder rotating at a constant speed and partially obstructing the measured flow. The rotating cylinder provides an auxiliary flow in the form of a part of the measured flow which rotates together with the cylinder (cf. FRG patent No. 1046350, Cl. 42e, 23/20). The differential pressure measured by this method is directly associated with the mass flow of a substance thus eliminating the need to measure its density. The differential pressure is proportional to the mass flow, hence, there are no limitations of the measurement range due to the square relationship of mass flow of a substance versus differential pressure in the flow.
There are, however, restrictions of the measurement range associated with the need in the interaction of boundary layers adjacent the relatively moving parts of a flow meter (in this case, rotating cylinder and flow meter body), thus resulting in rather small effective cross-sectional areas of flow meters.
Accuracy of measurement of the flow of a substance largely depends on the structure of the flow being measured as the points of pressure pick-up are located in the zone of interaction of measured and auxiliary flows during the measurement.
A change in structure of the measured flow may result in changes in or fluctuations of the differential pressure with unchanged flow rate of substance thus introducing an additional measurement error.